In certain applications involving the use of optical microscopes there arises a need to focus a small spot of light on to a specimen under observation. Possible uses include irradiating small specimens with laser light or trapping microparticles in optical gradient force traps. Such spots can be obtained by introducing light from a secondary source such as a laser into the optical path of the microscope and using the objective lens to focus the light. Additionally, some means of manipulating the position of the light spot within the field of view of the microscope should be provided.
Efficient focusing of light into a small spot requires utilization of the whole clear aperture of the lens used for the focusing. For example, in the case of a gaussian beam of light the size D of the spot will be: EQU D =4.lambda.f/.pi..phi. (1)
where
.lambda.--wavelength of light PA1 f--focal length of the lens PA1 .phi.--beam diameter at the lens.
A beam narrower than the aperture would not produce a spot of the minimum (i.e. diffracion limited) size. A wider beam would result in a loss of light power. These considerations apply regardless of the position of the spot, i.e. whether it is displaced away from the optical axis or not.